Interpretive Summary: Loss of nut yield and tree vigor due to parasitism of pecan trees by mistletoe is a major problem at certain locations within the southeastern sector of the U.S. pecan growing region. Pruning, biological, and chemical control methods have proven to be ineffective. An inexpensive and safe method is needed that kills mistletoe without harming host trees. The herbicide, 2,4-D, was found to be a safe and highly effective agent for control of mistletoe in dormant trees. The herbicide can potentially be used to effectively eradicate unwanted mistletoe from trees and orchards.

Technical Abstract:
Semi-parasitic evergreen mistletoe (Phoradendron flavescens Nutt.) is an increasingly serious weed causing loss of nut yield and tree vigor in pecan [Carya illinoinensis (Wangenh.) K. Koch] orchards of the southeastern U.S. Several herbicides and growth regulators were evaluated for efficacy against mistletoe. The dimethylamine salt of 2,4-D proved to be an effective control agent. Ethephone (2-chloroethyl) phosphonic acid, glyphosate, paraquat dichloride, and polyborate exhibited little or no long-term efficacy. The dimethylamine salt of dicamba also killed mistletoe, but exhibited potential for harming host trees. Dormant season treatment of mistletoe clusters with 2,4-D reduced photosynthesis by about one-third soon after treatment, and by about 90 percent from 6 to 16 weeks post-treatment, but clusters did not die until about 4 months post-treatment. Host limbs, less than about 3 cm in diameter at the site of mistletoe attachment, usually died within 12 months of 2,4-D treatment of the associated mistletoe cluster. Treatment of entire host trees with 2,4-D did not harm trees if applied prior to about 1 week of budbreak. Spot treatment of mistletoe clusters, with 2,4-D at 1.2 to 2.4 g a.i.L-1 (plus 2 percent crop oil), about 2 to 3 weeks before budbreak, gave effective long-term control of mistletoe. The inclusion of a crop-oil in the 2,4-D spray greatly increased efficacy.